## 20161017

Physics 205A, fall semester 2016
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing a presentation on collisions.

Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.
"This section was on collisions. There are three types, completely inelastic, partially inelastic, and elastic. Completely inelastic collisions will stick together. For inelastic collisions the two objects rebound off each other, and elastic collisions are when they rebound and there is no permanent damage. Elastic collisions are the only type where kinetic energy is conserved."

"There are three types of collisions in this section. Inelastic, in which two objects stick together, partially inelastic, in which two objects rebound off each other while inflicting damage, and elastic, in which two objects rebound off each other without causing permanent deformation."

"When a collision occurs and the objects stick together all of the kinetic energy is lost, this is complete inelasticity. If a collision occurs and there is some rebound off the other object and damage is smaller, then it is partially inelastic, but kinetic energy is still lost. When two objects bounce off of each other with no damage kinetic energy is conserved."

"It made sense to me how kinetic energy can only be conserved for elastic collisions; no damage is done in this type of collision and the two objects just bounce off one another, as opposed to crashing into one another and deforming each other in the two types of inelastic collisions."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.
"I found this section fairly straightforward. Momentum conservation was a little confusing to me, as well as using the calculation for it."

"The principle of conservation of linear momentum. More specifically, the breakdown of it and how it can be applied."

"Although I understand the concept of the collision types, the flow chart in the presentation sort of confused me. Is that just supposed to be a reference for help?"

Explain the difference between a (partially) inelastic collision and a completely inelastic collision.
"Partially inelastic collision the two objects rebound off each other and there is no permanent damage. With a completely inelastic collision, the objects stick together."

"In partially inelastic the cars are not stuck together like they are in completely inelastic. There is permanent damage in both these collisions."

"Completely inelastic collison is when most kinetic energy is lost, objects stick together and damage is done. Partially inelastic collison is when some kinetic energy is lost, damage is done but objects separate after."

Explain why drag, friction, and other external forces do not matter during sufficiently "brief" collisions, in order for momentum to be conserved.
"With car collisions, if the external friction forces are large, their impulse would be negligible due to the relatively brief time that collisions have."

"Their impulse would be negligible due to the relatively brief time that collisions take place in. We can set the change in time equal to zero, such that the objects are simply exchanging momentum."

"It's just the initial and final, nothing in-between?"

For the Nissan Altima and Nissan Rogue crash test, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)
 Completely inelastic. [0] (Partially) inelastic. ********************************** [34] Elastic. *** [3] (Unsure/lost/guessing/help!) ** [2]

For the Ford Explorer and Ford Taurus crash test, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)
 Completely inelastic. * [1] (Partially) inelastic. **** [4] Elastic. ******************************** [32] (Unsure/lost/guessing/help!) ** [2]

For the train and minivan crash, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)
 Completely inelastic. ***************************** [29] (Partially) inelastic. ***** [5] Elastic. [1] (Unsure/lost/guessing/help!) **** [4]

For the bullet burrowing through and back out of the baseball, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)
 Completely inelastic. ****** [6] (Partially) inelastic. ********************** [22] Elastic. [4] (Unsure/lost/guessing/help!) ******* [7]

For the bullet shot out of this gun, classify the type of collision. (Neglect drag/friction/external forces during this "brief" collision.)
 Completely inelastic. **** [4] (Partially) inelastic. ********** [10] Elastic. ******************** [20] (Unsure/lost/guessing/help!) ***** [5]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"For inelastic collisions, does the permanent damage have to be to both objects, or can there be permanent damage to just one of the objects? For the train-car example, since the train did not undergo any structural damage, is it is elastic for the train and inelastic for the car?" (Any permanent damage/deformation means that translational kinetic energy was lost, whether it was to one or both of the colliding objects, and so the collision would be classified as one of the two inelastic types.)

"Can we get some clarification what constitutes 'permanent damage?'" (Any evidence that shows that a collision took place. Be sure to ask your auto body specialist.)

For the elephant gun example, are we assuming there was no permanent deformation of the man's body (bruising, dislocated shoulder, etc.)? I don't know if translational kinetic energy was conserved to make this elastic or partially inelastic." (Consider the amount of translational kinetic energy before the gun was fired (zero, since everything was stationary), compared to afterwards, where the man, rifle, and bullet were all moving. Translational kinetic energy was definitely not conserved, as there is more afterwards than before. You can still say total energy was conserved, as all this new translational energy was released from the gunpowder's chemical energy, but translational kinetic energy by itself is not conserved.)

"I am somewhat confused about momentum. If impulse is equal to mass times the change in velocity, is momentum simply the change in velocity?" (Momentum is mass times velocity. Assuming that mass remains constant, then the change in momentum is the mass times the change in velocity.)